1. Field of the Invention
The present invention broadly relates to an apparatus for applying a coating powder to the surfaces of objects such as household electric appliances and other ordinary machines. More particularly, the present invention is concerned with a baking oven for heating a layer of powdered coating material electrostatically deposited on the surface of the object together with the object so as to form a coating film of the coating material.
2. Description of the Related Art
A typical conventional baking oven, known as mountain-shaped oven, has a tunnel-like flat heating chamber and inlet and outlet air-shield chambers connected to the inlet and outlet sides of the heating chamber so as to extend obliquely downward therefrom. A flexible conveyor is attached to the ceiling of these chambers so as to extend through these chambers. The inlet and outlet air shielding chambers which are extended obliquely downward effectively prevent hot air in the horizontal heating chamber from flowing outside by natural convection. In operation, the the object is heated by hot air from the room temperature to the temperature required for the baking of the coating material while it runs through upstream region of the space inside the horizontal heating chamber. Then the object is maintained at the baking temperature for a predetermined time which is necessary for baking the coating material, during running through the remainder region of the .space inside the heating chamber. In order to maintain the object at the baking temperature for the required length of time, it would be possible to reduce the running speed of the object. Such a reduced running speed, however, impractically hampers the mass-producibility of the product. Consequently, the mountain-type oven is required to have a considerably large length. When the length of the oven is limited, it is necessary to enhance the heat transfer rate by convection by increasing the number of the cycle of circulation of hot air in the oven. In case of the powder coating, the powder layer has not been fused in the region near the inlet of the furnace. The powder particles of the layer, therefore, are scattered when the velocity of the hot air is increased. Thus, the number of cycles of recirculation of hot air also is limited.